Re: byteswap.h behavior

[Jeffrey Walton <noloader@gmail.com>]

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On Sun, May 12, 2024 at 3:23 PM Collin Funk <collin.funk1@gmail.com> wrote:

> A few months ago there was a suggestion on emacs-devel to use
> __builtin_bswap functions when available [1]. While I agree that GCC
> can deal with the optimizations properly, I noted an important
> difference between the macros in byteswap.h.in and inline functions
> provided by glibc.
>
> Using this test program to compare the real glibc header to a macro
> copy-pasted from the replacement header:
>
> ======================================
> #define _GNU_SOURCE 1
> #include <stdio.h>
> #include <inttypes.h>
> #include <byteswap.h>
> #define bswap_32_macro(x) ((((x) & 0x000000FF) << 24) | \
>                            (((x) & 0x0000FF00) << 8) |  \
>                            (((x) & 0x00FF0000) >> 8) |  \
>                            (((x) & 0xFF000000) >> 24))
> int
> main (void)
> {
>   uint32_t value = 0x12345678;
>   uint32_t value_macro = 0x12345678;
>   printf ("1. %#" PRIX32 "\n", bswap_32 (value++));
>   printf ("2. %#" PRIX32 "\n", bswap_32_macro (value_macro++));
>   printf ("3. %#" PRIX32 "\n", value);
>   printf ("4. %#" PRIX32 "\n", value_macro);
>   return 0;
> }
> ======================================
>
> We get the output:
>
>     $ ./a.out
>     1. 0X78563412
>     2. 0X78563412
>     3. 0X12345679
>     4. 0X1234567C
>
> I would like to deal with this concern before I implement the
> replacement for <endian.h>. I think the best decision is to use
> 'extern inline' to match the behavior of glibc. Any other opinions on
> this?
>
> Also if we can agree upon making sure these are defined as functions,
> what is the proper way to test it in a configure script? My instinct
> tells me that assigning a function pointer to bswap_16, etc. would
> fail if they are macros but I am not sure the "standard" way of
> performing that check.
>
> Collin
>
> [1] https://lists.gnu.org/archive/html/emacs-devel/2024-03/msg00736.html
>

I think extern is the least of your worries. Pre- and post- increment are
not safe in a macro. Maybe you should make a local copy of the value in a
do {} while, and then operate on the local value. Or make it a function.

Jeff

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